Systems and methods for removing particles from a substrate processing chamber using rf plasma cycling and purging

ABSTRACT

Systems and methods for operating a substrate processing system include processing a substrate arranged on a substrate support in a processing chamber. At least one of precursor gas and/or reactive gas is supplied during the processing. The substrate is removed from the processing chamber. Carrier gas and purge gas are selectively supplied to the processing chamber. RF plasma is generated in the processing chamber during N cycles, where N is an integer greater than one. The RF plasma is on for a first period and off for a second period during each of the N cycles. The purge gas is supplied during at least part of each of the N cycles.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/297,745 filed Jun. 6, 2014. The entire disclosure of the applicationreferenced above is herein by reference.

FIELD

The present disclosure relates to substrate processing systems, and moreparticularly to systems and methods for removing particles from asubstrate processing chamber.

BACKGROUND

The background description provided here is for the purpose of generallypresenting the context of the disclosure. Work of the presently namedinventors, to the extent it is described in this background section, aswell as aspects of the description that may not otherwise qualify asprior art at the time of filing, are neither expressly nor impliedlyadmitted as prior art against the present disclosure.

Substrate processing systems may be used to perform deposition and/oretching of film on a substrate. Substrate processing systems typicallyinclude a processing chamber with a substrate support such as apedestal, an electrostatic chuck, a plate, etc. A substrate such as asemiconductor wafer may be arranged on the substrate support. Inchemical vapor deposition (CVD) or atomic layer deposition (ALD)processes, a gas mixture including one or more precursors may beintroduced into the processing chamber to deposit a film on thesubstrate. In some substrate processing systems, radio frequency (RF)plasma may be used to activate chemical reactions.

Some chemical reactions that happen in the gas phase generate particlesthat may remain in the processing chamber after processing is completed.In addition to particles created during processing, particles may alsoreach the processing chamber due to dusted upstream parts, chamber leakevents, contamination that occurs when replacing parts, and/orcontamination that occurs during maintenance.

In some processes, purge gas is cycled on and off after the substratesare removed from the processing chamber to remove particles remaining inthe processing chamber. Removing particles using purge gas cycling takesa relatively long time (˜24 hours) and may not reduce particles in theprocessing chamber to an acceptable level.

SUMMARY

A method for operating a substrate processing system includes a)processing a substrate arranged on a substrate support in a processingchamber, wherein at least one of precursor gas and/or reactive gas issupplied during the processing; b) removing the substrate from theprocessing chamber; c) selectively supplying carrier gas and purge gasto the processing chamber; d) generating RF plasma in the processingchamber during N cycles, where N is an integer greater than one, whereinthe RF plasma is on for a first period and off for a second periodduring each of the N cycles; and e) supplying the purge gas during atleast part of each of the N cycles of the RF plasma

In other features, the purge gas is not supplied during the first periodand is supplied during at least part of the second period of the Ncycles. The at least one of the precursor gas and/or the reactive gas isnot supplied during (c), (d) or (e). (a) comprises depositing film usingRF plasma. (a) comprises one of atomic layer deposition (ALD) andchemical vapor deposition (CVD).

In other features, the one of ALD and CVD employs RF plasma. A dutycycle of the N cycles is between 25% and 75%. (c) is performed during(d) and (e).

In other features, each of the N cycles has a duration between 1 and 5seconds. N is greater than or equal to 100 and less than or equal to5000. At least one of a duty cycle of the N cycles and/or a duration ofthe N cycles is varied during the N cycles.

A substrate processing system includes a processing chamber including asubstrate support to support a substrate during processing. A gas supplyselectively supplies carrier gas, purge gas and at least one ofprecursor gas and/or reactive gas during the processing. A controller isconfigured to a) supply the carrier gas to the processing chamber afterthe substrate is removed from the processing chamber; b) generate RFplasma in the processing chamber during N cycles, where N is an integergreater than one, wherein the RF plasma is on for a first period and offfor a second period during each of the N cycles; and c) supply the purgegas during at least part of each of the N cycles of the RF plasma.

In other features, the purge gas is not supplied during the first periodand is supplied during at least part of the second period of the Ncycles.

In other features, the controller is configured to not supply theprecursor gas and the reactive gas during (a), (b) and (c). Thesubstrate processing system deposits film using RF plasma. The substrateprocessing system performs one of atomic layer deposition (ALD) andchemical vapor deposition (CVD). The one of ALD and CVD employs RFplasma. The controller is configured to control a duty cycle of the Ncycles between 25% and 75%.

In other features, the controller is configured to supply the carriergas during (b) and (c). Each of the N cycles has a duration between 1and 5 seconds. N is greater than or equal to 100 and less than or equalto 5000. At least one of a duty cycle of the N cycles and/or a durationof the N cycles is varied during the N cycles.

A method for operating a substrate processing system includes a)removing a substrate from a substrate support in a processing chamber;b) selectively supplying carrier gas and purge gas to the processingchamber; c) generating RF plasma in the processing chamber during Ncycles, where N is an integer greater than one, wherein the RF plasma ison for a first period and off for a second period during each of the Ncycles; and d) supplying the purge gas during at least part of each ofthe N cycles of the RF plasma.

In other features, the purge gas is not supplied during the first periodand is supplied during at least part of the second period of the Ncycles.

A substrate processing system includes a processing chamber including asubstrate support to support a substrate during processing. A gas supplysupplies carrier gas and purge gas. A controller is configured to a)supply the carrier gas to the processing chamber after the substrate isremoved from the processing chamber; b) generate RF plasma in theprocessing chamber during N cycles, where N is an integer greater thanone, wherein the RF plasma is on for a first period and off for a secondperiod during each of the N cycles; and c) supply the purge gas duringat least part of each of the N cycles of the RF plasma.

In other features, the purge gas is not supplied during the first periodand is supplied during at least part of the second period of the Ncycles.

Further areas of applicability of the present disclosure will becomeapparent from the detailed description, the claims and the drawings. Thedetailed description and specific examples are intended for purposes ofillustration only and are not intended to limit the scope of thedisclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a functional block diagram of an example of a substrateprocessing system according to the present disclosure;

FIG. 2 is a flowchart illustrating an example of a method for performingatomic layer deposition (ALD);

FIG. 3 is a flowchart illustrating an example of a method for removingparticles from a substrate processing chamber according to the presentdisclosure;

FIG. 4 is a flowchart illustrating another example of a method forremoving particles from a substrate processing chamber according to thepresent disclosure; and

FIGS. 5 and 6 are graphs illustrating timing of control signals forsupplying precursor, reactive gas, carrier gas, purge gas, and RF plasmawhen removing particles from the processing chamber according to thepresent disclosure.

In the drawings, reference numbers may be reused to identify similarand/or identical elements.

DETAILED DESCRIPTION

Cycling purge gas to remove particles in the processing chamber is notvery efficient. The relatively low efficiency of purge gas in reducingparticles in the processing chamber may be due in part to electrostaticforce, which causes particles to stick on surfaces of the processingchamber. In addition, particles may also congregate due to electrostaticforce and may be trapped inside components of the processing chambersuch as a showerhead. Particles that are held by electrostatic force aredifficult to remove by cycling purge gas.

The present disclosure relates to systems and methods for removingparticles from processing chambers. As disclosed herein, after thesubstrate is removed, particles in the processing chamber may be removedby cycling RF plasma and purge gas. In some examples, the timing of theRF plasma cycles is similar to cycling that is used during ALD filmdeposition.

In some examples, reactant gases and precursor are not supplied duringremoval of the particles. Carrier gases are supplied and RF plasma iscycled on and off. Continuous or pulsed purge gas is used to removeparticles from the processing chamber.

In some examples, the systems and methods according to the presentdisclosure may be used to remove particles from processing chambers usedto deposit film by ALD or PEALD processes. Example film types includeSiO₂, SiN, SiCN, SiC, noble metals, and high K materials includinglanthanide-oxides, group 4 metals oxides and group 5 metal oxides,although other types of film and/or other processes may be involved. Forexample, the present disclosure may also be used to remove particlesfrom processing chambers used to deposit film by CVD or PECVD processes.Example film types include SiO₂, SiN, TEOS, SiC, SiCN, and AHM, althoughother types of film and/or processes may be used.

According to the present disclosure, the substrate is removed afterprocessing, carrier gas is supplied and RF power is cycled on and off tostrike RF plasma. In some examples, the RF cycling may have similartiming as that used in ALD film deposition. In some examples, thesubstrate is removed using a robot or an indexing mechanism. Precursorand reactive gases are not supplied during the RF cycling. During the RFcycling, continuous or pulsed purge gas may be used to remove theparticles from the processing chamber.

The RF cycling and purging may help to release particles that areelectrostatically stuck on surfaces in the processing chamber. Theparticles are released during the RF cycling and exit the processingchamber along with the purge gas. In addition, particles charged by theRF plasma repel each other such that particle congregation can be drawnby the purge gas flow out of the processing chamber.

Advantages of RF cycling and purging according to the present disclosureinclude significantly reducing the time required to lower the particlecount in the process chamber below a predetermined value. For example,2-3 hours of RF cycling and purging as described herein may have betterparticle reduction performance than 24-48 hours of gas-only cyclepurging. As a result, the processing chamber post-maintenanceconditioning time is significantly reduced.

Referring now to FIG. 1, an example of a substrate processing system 10for removing mechanical particles using RF cycling and purging is shown.The substrate processing system 10 includes a processing chamber 12. Gasmay be supplied to the processing chamber 12 using a gas distributiondevice 14 such as showerhead or other device. A substrate 18 such as asemiconductor wafer may be arranged on a substrate support 16 duringprocessing. The substrate support 16 may include a pedestal, anelectrostatic chuck, a mechanical chuck or other type of substratesupport.

A gas delivery system 20 may include one or more gas sources 22-2, 22-2,. . . , and 22-N (collectively gas sources 22), where N is an integergreater than one. Valves 24-1, 24-2, . . . , and 24-N (collectivelyvalves 24), mass flow controllers 26-1, 26-2, . . . , and 26-N(collectively mass flow controllers 26), or other flow control devicesmay be used to controllably supply precursor, reactive gases, inertgases, purge gases, and mixtures thereof to a manifold 30, whichsupplies the gas mixture to the processing chamber 12.

A controller 40 may be used to monitor process parameters such astemperature, pressure etc. (using sensors 41) and to control processtiming. The controller 40 may be used to control process devices such asthe gas delivery system 20, a pedestal heater 42, and/or a plasmagenerator 46. The controller 40 may also be used to evacuate theprocessing chamber 12 using a valve 50 and pump 52.

The RF plasma generator 46 generates the RF plasma in the processingchamber. The RF plasma generator 46 may be an inductive orcapacitive-type RF plasma generator. In some examples, the RF plasmagenerator 46 may include an RF supply 60 and a matching and distributionnetwork 64. While the RF plasma generator 46 is shown connected to thegas distribution device 14 with the pedestal grounded or floating, theRF generator 46 can be connected to the substrate support 16 and the gasdistribution device 14 can be grounded or floating.

Referring now to FIG. 2, an example of a method for performing atomiclayer deposition (ALD) is shown. While an ALD process is shown forillustration purposes, the systems and methods described herein canapply to other types of processes including but not limited to CVD,PECVD, PEALD, etc.

At 104, a substrate is arranged in a processing chamber. Processes gasessuch as one or more carrier gases or inert gases may be supplied to theprocessing chamber. At 106, a first precursor is supplied to theprocessing chamber for a first period. After the first period, theprocessing chamber is purged at 110. At 114, a second precursor may besupplied to the processing chamber for a second period to react with thefirst precursor. Alternately or additionally, RF plasma may be struck inthe processing chamber to convert the first precursor. After a secondperiod, the processing chamber is purged at 116. At 120, one or moreadditional ALD cycles are performed and control returns to 106.Otherwise, when the ALD cycles are complete, control ends. Whenprocessing is completed, the substrate is removed from the processingchamber.

Referring now to FIG. 3, an example of a method for removing particlesfrom a substrate processing chamber is shown. The systems and methodsdescribed herein are performed after the substrate is removed from theprocessing chamber. In this example, the purge gas remains on during theRF plasma cycling and purging. At 204, the substrate is removed from theprocessing chamber. At 206, carrier gas is supplied to the processingchamber. At 210, purge gas is supplied to the processing chamber. At214, RF plasma is struck for a predetermined period. At 216, the RFplasma is extinguished after the predetermined period. When additionalRF plasma cycles are to be performed (as determined at 220), controlcontinues with 206. When the predetermined number of cycles iscompleted, control ends.

Referring now to FIG. 4, another example of a method for removingparticles from a substrate processing chamber is shown. In this example,the purge gas is cycled on and off after the RF signal is turned off. At304, a substrate is removed from the processing chamber. At 306, carriergas is supplied to the processing chamber. At 314, RF plasma is struckfor a first predetermined period. At 316, the RF plasma is extinguishedafter the first predetermined period. At 318, the processing chamber ispurged with purge gas for a second predetermined period after the RFplasma is extinguished. If additional RF plasma and purge cycles areneeded, control continues with 306. When a sufficient number of cyclesare completed, control ends.

Referring now to FIGS. 5 and 6, graphs illustrating examples of timingof control signals for supplying precursor, reactive gas, carrier gas,purge gas, and RF are shown. The timing shown in FIG. 5 corresponds tothe method described in FIG. 3. The precursor and reactive gases are notsupplied to the processing chamber. Carrier gas and purge gas aresupplied during RF pulses.

The timing shown in FIG. 6 corresponds to the method described in FIG.4. The precursor and reactive gases are not supplied to the processingchamber during the RF plasma cycling and purging. Carrier gas issupplied during the RF chucking. The purge gas is pulsed for apredetermined period after falling edges of the RF pulses. While thepurge gas is shown as stopping a predetermined period before leadingedges of a subsequent RF pulse, the purge gas may also be supplied untilthe leading edges or just after the leading edges of the subsequent RFpulses. In some examples, a duty cycle of the RF cycling is between 25%and 75%. In other examples, the duty cycle and the period can be variedduring particle removal. Varying the duty cycle and/or duration may helpto dislodge particles. In some examples, the RF cycles are 1 to 5seconds long, although other durations may be used. In some examples,100 to 5000 cycles are performed, although additional or fewer cyclesmay be used. In some examples, 2000 to 3000 cycles are performed,although additional or fewer cycles may be used.

In one example, the processing chamber was dusted after a chamber leakevent. 10 hours of purge gas-only cycling reduced the chamber mechanicalparticles to ˜1000 adders. This approach required another 22 hours ofpurge gas-only cycling to further reduce the chamber mechanicalparticles to <30 adders @ 0.06 μm as shown in Table I:

TABLE I Particle Tests Particle adders Tool recovery after leakevent >0.06 μm >0.12 μm Run gas-only cycle purge for 10 hours Mechanicalgas only particle test 400 Cycles 1141 213 Run gas-only cycle purge foranother 10 hours Mechanical gas only particle test 400 Cycles 478 101Run gas-only cycle purge for another 12 hours Mechanical gas onlyparticle test 400 Cycles 14 6

In another example, the substrate processing tool suffered high particlecount for an ALD Ox process after a showerhead leak issue. Using 24 hourpurge gas-only cycling, the processing chamber mechanical particle countwas still at ˜100. Next, using RF cycling and purging for 2 hours, theprocessing chamber mechanical particle count dropped to ˜30 adders.Another 1 hour of RF cycling and purging further reduced the mechanicalparticle count as shown in Table II:

TABLE II Tool recovery from leak 0.04 μm 0.05 μm 0.06 μm 0.08 μm 0.1 μmALD Ox process A 100A 1434 1048 924 527 303 Purge gas cycling 24 hoursMechanical gas only 145 94 73 38 20 particle test RF chucking purge 2hours Mechanical gas only 33 27 23 14 6 particle test RF chucking purge1 hour Mechanical gas only 15 11 10 6 3 particle test

In another example, the RF cycling and purging for 1 hour significantlyreduced the 2000 Angstrom in-film particle count from ˜800 adders to˜100 adders and reduced the processing chamber mechanical particlecount. An additional 2 hours of RF cycling and purging further reducedthe 2000 A in-film particle count as shown in Table III:

TABLE III Particle Tests 0.06 μm 0.08 μm 0.1 μm 0.12 μm 0.16 μm 0.2 μm0.5 μm ALD Ox process B 2000A 842 448 212 23 (2800 cycles) Cycle purgewith RF-chucking for 1 hour Mechanical gas only 30 18 8 8 6 5 2 particletest (2800 cycles) ALD Ox process B 2000A 130 40 20 7 (2800 cycles)Cycle purge with RF-chucking for 2 hours ALD Ox process B 2000A 34 11 95 (2800 cycles)

The foregoing description is merely illustrative in nature and is in noway intended to limit the disclosure, its application, or uses. Thebroad teachings of the disclosure can be implemented in a variety offorms. Therefore, while this disclosure includes particular examples,the true scope of the disclosure should not be so limited since othermodifications will become apparent upon a study of the drawings, thespecification, and the following claims. As used herein, the phrase atleast one of A, B, and C should be construed to mean a logical (A OR BOR C), using a non-exclusive logical OR, and should not be construed tomean “at least one of A, at least one of B, and at least one of C.” Itshould be understood that one or more steps within a method may beexecuted in different order (or concurrently) without altering theprinciples of the present disclosure.

In this application, including the definitions below, the termcontroller may be replaced with the term circuit. The term controllermay refer to, be part of, or include: an Application Specific IntegratedCircuit (ASIC); a digital, analog, or mixed analog/digital discretecircuit; a digital, analog, or mixed analog/digital integrated circuit;a combinational logic circuit; a field programmable gate array (FPGA); aprocessor circuit (shared, dedicated, or group) that executes code; amemory circuit (shared, dedicated, or group) that stores code executedby the processor circuit; other suitable hardware components thatprovide the described functionality; or a combination of some or all ofthe above, such as in a system-on-chip.

The controller may include one or more interface circuits. In someexamples, the interface circuits may include wired or wirelessinterfaces that are connected to a local area network (LAN), theInternet, a wide area network (WAN), or combinations thereof. Thefunctionality of any given controller of the present disclosure may bedistributed among multiple controllers that are connected via interfacecircuits. For example, multiple controllers may allow load balancing. Ina further example, a server (also known as remote, or cloud) controllermay accomplish some functionality on behalf of a client controller.

The term code, as used above, may include software, firmware, and/ormicrocode, and may refer to programs, routines, functions, classes, datastructures, and/or objects. The term shared processor circuitencompasses a single processor circuit that executes some or all codefrom multiple controllers. The term group processor circuit encompassesa processor circuit that, in combination with additional processorcircuits, executes some or all code from one or more controllers.References to multiple processor circuits encompass multiple processorcircuits on discrete dies, multiple processor circuits on a single die,multiple cores of a single processor circuit, multiple threads of asingle processor circuit, or a combination of the above. The term sharedmemory circuit encompasses a single memory circuit that stores some orall code from multiple controllers. The term group memory circuitencompasses a memory circuit that, in combination with additionalmemories, stores some or all code from one or more controllers.

The term memory circuit is a subset of the term computer-readablemedium. The term computer-readable medium, as used herein, does notencompass transitory electrical or electromagnetic signals propagatingthrough a medium (such as on a carrier wave); the term computer-readablemedium may therefore be considered tangible and non-transitory.Non-limiting examples of a non-transitory, tangible computer-readablemedium include nonvolatile memory circuits (such as a flash memorycircuit or a mask read-only memory circuit), volatile memory circuits(such as a static random access memory circuit and a dynamic randomaccess memory circuit), and secondary storage, such as magnetic storage(such as magnetic tape or hard disk drive) and optical storage.

The apparatuses and methods described in this application may bepartially or fully implemented by a special purpose computer created byconfiguring a general purpose computer to execute one or more particularfunctions embodied in computer

Attorney Docket No. 3272-2US programs. The computer programs includeprocessor-executable instructions that are stored on at least onenon-transitory, tangible computer-readable medium. The computer programsmay also include or rely on stored data. The computer programs mayinclude a basic input/output system (BIOS) that interacts with hardwareof the special purpose computer, device drivers that interact withparticular devices of the special purpose computer, one or moreoperating systems, user applications, background services andapplications, etc. The computer programs may include: (i) assembly code;(ii) object code generated from source code by a compiler; (iii) sourcecode for execution by an interpreter; (iv) source code for compilationand execution by a just-in-time compiler, (v) descriptive text forparsing, such as HTML (hypertext markup language) or XML (extensiblemarkup language), etc. As examples only, source code may be written inC, C++, C#, Objective-C, Haskell, Go, SQL, Lisp, Java®, ASP, Perl,Javascript®, HTML5, Ada, ASP (active server pages), Perl, Scala, Erlang,Ruby, Flash®, Visual Basic®, Lua, or Python®.

None of the elements recited in the claims is intended to be ameans-plus-function element within the meaning of 35 U.S.C. §112(f)unless an element is expressly recited using the phrase “means for”, orin the case of a method claim using the phrases “operation for” or “stepfor”.

What is claimed is:
 1. A substrate processing system comprising: aprocessing chamber including a substrate support to support a substrateduring processing; a gas supply to selectively supply carrier gas, purgegas, and at least one gas selected from a group including precursor gasand reactive gas during the processing; a radio frequency (RF) plasmagenerator to selectively generate RF plasma in the processing chamber;and a controller that communicates with the gas supply and the RF plasmagenerator and that is configured to: a) supply the carrier gas to theprocessing chamber after the substrate is removed from the processingchamber; b) generate RF plasma in the processing chamber during Ncycles, where N is an integer greater than one, wherein the N cyclesinclude alternating first periods and second periods, and wherein the RFplasma is on for the first periods and off for the second periods duringthe N cycles; c) not supply the purge gas during the first periods ofthe N cycles when the RF plasma is on; and (d) supply the purge gasduring the second periods of the N cycles when the RF plasma is off. 2.The substrate processing system of claim 1, wherein the controller isconfigured to not supply the at least one gas selected from the groupduring (a), (b), (c) and (d).
 3. The substrate processing system ofclaim 1, wherein the substrate processing system deposits film using RFplasma.
 4. The substrate processing system of claim 1, wherein thesubstrate processing system performs one of atomic layer deposition(ALD) and chemical vapor deposition (CVD).
 5. The substrate processingsystem of claim 4, wherein the one of ALD and CVD employs RF plasma. 6.The substrate processing system of claim 1, wherein the controller isconfigured to control a duty cycle of the N cycles between 25% and 75%.7. The substrate processing system of claim 1, wherein the controller isconfigured to supply the carrier gas during (b), (c) and (d).
 8. Thesubstrate processing system of claim 1, wherein each of the N cycles hasa duration between 1 and 5 seconds.
 9. The substrate processing systemof claim 1, wherein N is greater than or equal to 100 and less than orequal to
 5000. 10. The substrate processing system of claim 1, whereinat least one of a duty cycle of the N cycles and/or a duration of the Ncycles is varied during the N cycles.
 11. A substrate processing systemcomprising: a processing chamber including a substrate support tosupport a substrate during processing; a gas supply to selectivelysupply carrier gas, purge gas, and at least one gas selected from agroup including precursor gas and reactive gas during the processing; aradio frequency (RF) plasma generator to selectively generate RF plasmain the processing chamber; and a controller that communicates with thegas supply and the RF plasma generator and that is configured to: a)supply the carrier gas to the processing chamber after the substrate isremoved from the processing chamber; b) generate RF plasma in theprocessing chamber during N cycles, where N is an integer greater thanone, wherein the N cycles include alternating first periods and secondperiods, and wherein the RF plasma is on for the first periods and offfor the second periods during the N cycles; c) not supply the purge gasduring the first periods of the N cycles when the RF plasma is on; and(d) supply the purge gas during the second periods of the N cycles whenthe RF plasma is off, wherein the substrate processing system performsone of atomic layer deposition (ALD) and chemical vapor deposition (CVD)and wherein N is greater than or equal to 100 and less than or equal to5000.
 12. The substrate processing system of claim 11, wherein thecontroller is configured to not supply the at least one gas selectedfrom the group during (a), (b), (c) and (d).
 13. The substrateprocessing system of claim 11, wherein the one of ALD and CVD employs RFplasma.
 14. The substrate processing system of claim 11, wherein thecontroller is configured to control a duty cycle of the N cycles between25% and 75%.
 15. The substrate processing system of claim 11, whereinthe controller is configured to supply the carrier gas during (b), (c)and (d).
 16. The substrate processing system of claim 11, wherein eachof the N cycles has a duration between 1 and 5 seconds.
 17. Thesubstrate processing system of claim 11, wherein at least one of a dutycycle of the N cycles and/or a duration of the N cycles is varied duringthe N cycles.